High sensitivity piezoelectric microphone



-13, 1945sv F. SWINEHART my. 3,100,821

HIGH SENSITIVITY PIEZOELECTRIC MICROPHONE Fil ed March a, 1961 27 INKENTORS fi-am darmefiarlf ,2040, v/dores Mas ATTORNEYS United States Patent The present invention relates to improvements in piezoelectric type microphones for the purpose of improving the voltage output of such microphones, and in improving their frequency response.

While the present invention has applicability to all types of piezoelectric microphone assemblies, it finds particular utility in conjunction with ceramic elements.

Such elements have desirable characteristics when employed in piezoelectric type microphones, but, in general,

their voltage output is significantly lower than that obtainable from the use of other piezoelectric elements such as Rochelle salt crystals. I

A typical ceramic element for use in microphone assemblies consists of an elongated body of ceramic material having opposed faces covered with silver or other electrical conductors. The element is actuated by coupling one end of the element by means of a pin or the like to a flexible diaphragm which responds to the sound waves energizing the microphone. The mechanical vibration of the diaphragm then sets up stresses in the element which are converted to electrical signals by the element.

The problem of securing the ceramic element in the micr'ophoneassembly has always been substantial. The

most common method in use today consists in providing a two-point suspension in which a substantial amount of the element is clamped, by spring pressure or otherwise, along two spaced areas of the element.

We have found that this type of suspension seriously affects the performance of the piezoelectric element. The portion of the element clamped between the two points of suspension is practically useless as far as generating voltage is concerned, but adds a significant amount of capacitive reactance to the electrical circuit of the microphone. The net result is a generally low' output level and impaired frequency response. Since the portion of the element between the points of suspension is practically useless, and this length may constitute close to onehalf the length of the element, the cost of the unit is quite high in relation to its performance characteristics.

A principal object of the present invention is to provide an improved piezoelectric type microphone assembly I having a higher voltage output while using a smaller element than heretofore.

A further object of the inventionis to provide an improved piezoelectric type microphone in which the catilever suspension within the microphone housing. With this type of mounting, there is practically no length of piezoelectric element which does not contribute to the voltage output, and the overall capacitance of the element is reduced in comparison with those assemblies which employ the two-point suspension system. With the improved structure of the present invention, we are able to provide a microphone which has an output level averaging 4 to 6 dbabove microphones employing thetwopoint suspension, while using a shorter element than pre-- viously, and without suffering adverse effects upon the frequency response. 4

A further description of the present invention will be made in conjunction with the attached sheet of drawings which illustrates a preferred embodiment thereof.

. In the drawings:

FIGURE 1 is a plan view of the microphone assembly, with the diaphragm removed to illustrate the interior construction thereof;

FIGURE 2 isa cross-sectional view of the microphone assembly with the diaphragm in place;

FIGURE 3 is a circuit diagram illustrating the equivalent electrical circuit for piezoelectric type microphones; and

FIGURE 4 is a plan view of the support element employed in the structure of FIGURES 1 and 2.

As shown in the drawings:

In FIGURE 1, reference numeral 10 indicates general- 1y a thin walled housing having an annular flange portion 11, a cylindrical wall portion 12, and a circular base portion 13 as illustrated in FIGURE 2.

A ceramic piezoelectric element 15 is provided, the element 15 consisting of a ceramic body having its opposed faces coated with silver or other good conductor to constitute metallic electrodes at these opposed faces. The element 15 is energized in the conventional fashion by coupling it to a thin metallic diaphragm 16 by means of a pin 17. Thus, vibration of the diaphragm 16 by the sound waves stresses the ceramic element 15 and produces a voltage across its opposed electrode faces.

In order to describe more completely the effect which is achieved in the present invention, we have illustrated the equivalent electrical circuit of the microphone assembly in FIGURE 3. The ceramic piezoelectric element 15 constitutes the equivalent of a low impedance generator 18. The capacity of the element 15 is represented by a capacitor 19. When the element is included in a circuit, there is an additional capacity represented bythe capacitor 21 placed across the output of the generator 18 from the capacity of the cable and similar factors. The actual output voltage of the generator 18 which would appear across terminals 22 and 23 therefore varies inversely as the reactance provided by the capacitor 19. The reduction of that reactance is accomplished by the improved suspension means of the present invention.

As illustrated best in FIGURES 2 and 4, the suspension means of the present invention provide a support which holds the element along a single region of limited area along the end of the crystal element in cantilever fashion. A lower support element, generally indicated at numeral 24 in the drawings is secured to the base 13 by means of a bolt 26 and a nut (not shown) at the opposite side of the base 13 by means of which electrical connection can be made to the lower support 24.

The support 24 has a raised portion 27 having a recess 28 formed therein to receive the end portion of the crystal element 15.

The element 15 is clamped within the recess 28 by means of an electrically conductive arm 29 which overlies the portion of the element 15 which is seated in the recess 28, as illustrated best in FIGURE 2. The arm 29 is electrically insulated from the support 24 but is in A nut 34 is employed to apply clamping pressure to the Patented Aug. 13, 1963" arm 29, and providea convenient means for attaching a lug 36 for making the necessary electrical connections.

from the microphone assembly.

In the event that the input impedance of the associated amplifying or electronic equipment makes it necessary or desirable, it is sometimes desirable to include a hollow tube 38 in the housing for the purpose of acoustically resonating the cavity of the microphone behind the diaphragm 16 The tube 38 extends throughthe cylindrical wall 12 and is secured thereto by means of a suitable adhesive 39.

With a microphone assembly of the presentinvention, tests have indicated that the capacity of the element changed from 520 micromicrofarads to 360 micromicro farads when the conventional two-point suspension system was replaced by the improved suspension system illustrated in the drawings. This decrease in capacity was accompanied by a gain of 4 to 6 db in voltage output, despite the fact that the element employed in the improved suspension system was only about one-half the length of the element employed previously.

It should be evident, therefore, that the improved We claim as our invention: A piezoelectric type microphone comprising a housing, a support means in said housing, said support means including a recess on one surface thereof, a diaphragm' closing one end of said housing, an elongated piezoelectric element having one end portion snugly received within said recess and its opposite end portion coupled to said diaphragm, a grommet extending through said support means and securing said support means within said housing, a pressure applying arm engaging the end portion of said piezoelectric element which is received in said recess, a bolt arranged to apply clamping pressure to said arm and extending through said grommet, and a nut threaded on said bolt to provide an adjustable pressure on said arm. 7

References Cited in the file of this patent UNITED STATES PATENTS Germany Apr. 30, 

